Heat gun

ABSTRACT

Hand held heat gun producing heated air in 250* to 1,000* F range employs high performance internal combustion burner discharging exhaust gas at velocity above 4,000 feet per minute and temperature on order to of stoichiometric burning temperature, i.e. 3,450* F for propane fuel. High velocity exhaust gases enter mixing zone preferably in a divergent manner and with perimeter of gas flow cross-section at least 25 percent greater than the perimeter of a circle of equal area, to provide an extended gas-air interface The exhaust gas has a high volume pumping and mixing action upon ambient air, producing in a practical small distance a useful flow of treating air at the desired temperature. Preferably the burner outlet is of elongated form with decreasing cross-section towards outlet, e.g. a multiple legged outlet cross-section. Another form has a showerhead like series of small outlets, with diverging axes. Preferably a positioning means set a minimum distance between work piece and outlet, to ensure mixing. Where the exhaust gas stream is exposed to admission of increasing air along the length, the positioning means sets the working temperature. Preferably a shield extends about the exhaust gas stream, preferably in the form of a tube with space for ambient air. With apertures along the tube length, the mass of air increases in mass and decreases rapidly in temperature there along, so length sets discharge temperature. With a closed wall tube the tube cross-section and its inlet in the vicinity of the burner outlet defines the amount of ambient air entrained and thereby sets the discharge temperature. Burners useful in this heat gun are of high capacity type with jet pump feed, pressure recovery passage and flame holder positioned at entry of fuel mixture into the burner chamber.

United States Patent 91 Zagoroff Dec. 18, 1973 1 1 HEAT GUN [76]Inventor: Dimiter S. Zagoroff, 13 Cliff St.,

Marblehead, Mass. 01947 [22] Filed: Nov. 10, 1971 [21] Appl. No.:197,207

[52] US. Cl 431/347, 431/158, 431/352, 431/353 [51] Int. Cl. F23d 13/12,F23d 15/02 [58] Field of Search 431/158, 347, 351-355; 126/226, 229,231, 233, 237, 238, 401, 403, 406

[56] References Cited UNITED STATES PATENTS 2,578,101 12/1951 Stalego431/158 2,746,529 5/1956 Kamm et al. 431/158 2,398,654 4/1946 Lubbock et211.... 431/352 1,921,152 8/1933 Caldwell 431/158 3,506,198 4/1970 VanDer Zwaal 431/354 3,385,381 5/1968 Calaman 431/158 2,001,739 5/1935MacGregor.... 431/352 1,925,183 9/1933 Forster 431/353 3,299,940 1/1967Phillips et a1. 431/347 3,574,506 4/1971 Locke 431/344 2,666,480 l/l954Peterson 431/344 Primary Examiner-Carroll B. Dority, Jr. Attorney-JohnNoel Williams [57] ABSTRACT Hand held heat gun producing heated air in250 to l,000 F range employs high performance internal combustion burnerdischarging exhaust gas at velocity above 4,000 feet per minute andtemperature on order to of stoichiometric burning temperature, i.e.3,450 F for propane fuel. High velocity exhaust gases enter mixing zonepreferably in a divergent manner and with perimeter of gas flowcross-section at least 25 percent greater than the perimeter of a circleof equal area, to provide an extended gas-air interface The exhaust gashas a high volume pumping and mixing action upon ambient air, producingin a practical small distance a useful flow of treating air at thedesired temperature. Preferably the burner outlet is of elongated formwith decreasing cross-section towards outlet, e.g. a multiple leggedoutlet cross-section. Another form has a shower-head like series ofsmall outlets, with diverging axes. Preferably a positioning means set aminimum distance between work piece and outlet, to ensure mixing. Wherethe exhaust gas stream is exposed to admission of increasing air alongthe length, the positioning means sets the working temperature.Preferably a shield extends about the exhaust gas stream, preferably inthe form of a tube with space for ambient air. With apertures along thetube length, the mass of air increases in mass and decreases rapidly intemperature there along, so length sets discharge temperature. With aclosed wall tube the tube cross-section and its inlet in the vicinity ofthe burner outlet defines the amount of ambient air entrained andthereby sets the discharge temperature. Burners useful in this heat gunare of high capacity type with jet pump feed, pressure recovery passageand flame holder positioned at entry of fuel mixture into the burnerchamber.

7 Claims, 14 Drawing Figures .pmmmnmama I 3.179594 sum aura /l/lllllynn'n "I;

I, il llllnnnnnnh q l A jam; JET 1 l B UCTED JET TEMPERATUREunulullnlllnllnulllal FT/MIN' VELOCITY LBS/MIN Q .l o

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HEAT GUN Numerous applications in industry and home require lowtemperature heating, for example heating of plastics, to shrink film andto weld and soften tubing, and putty and paint, to soften and remove ordry. Low temperature, in the range of 250 to l,000 F, is extremelyimportant since higher temperatures lead to blistering, cracking andcharring of these inherently low temperature materials. The most widelyused tool for this purpose is the electric heat gun. An electric blowerpasses cold air over a resistance heating element, and the hot air isdirected at the work piece. Two disadvantages are that the power islimited to 3 kw using common electric outlets rated for 30 amp fuses andthe tool is not usable in the field where electricity is not available.

To get around the first limitation, units have been built in which a gasflame supplies the heat and a blower is used to mix in tempering air.These units, incorporating two different power systems, are relativelycomplicated, bulky, and expensive. A typical 25 kw unit intended forhand held use weighs 12 lbs.

Units that rely on fuel alone, such as hand held torches have theproblem that the flame temperature of common fuels such as natural gasor propane are quite high, above 3,000 F, many more times the desiredtemperature. In an attempt to avoid overheating the product, effortshave been made to slow down the flame being applied to the work piece bymeans such as spreaders, or by employing fuel rich, so-called yellowflames, but still hot spots, overheating, scorching and charringproblems persist.

I have discovered that I can achieve a much more satisfactory lowtemperature heating device capable of use as a tempered air heat gunusing a flame alone by employing high velocity, intense burning ratherthan the common low velocity, diffused burning pattern. This findingappears at first sight contradictory. Commonly, higher velocity burnersare employed to achieve faster, more intense heating rates. Thisbehaviour can be illustrated by plotting the time needed to startmelting the end at e.g. a copper piece using the same energy input butvarying the exhaust gas velocity directed against the copper. Theinference here is that when more gentle heating is sought, lowvelocities should be employed. The gas torches described above attemptto employ this principle to achieve gentle heating.

l have realized the practical importance in this context of the factthat, provided the work piece is held some distance away from theburner, quite beyond the flame, extremely high velocities lead to moregentle and uniform heating which can be controlled to the degreerequired. This behaviour can be illustrated as a plot of the peaktemperature of the products measured a distance away from a 15,000Btu/hr burner as a function of burner exhaust gas velocity. If atemperature of 1,000 F is sought within a distance of 9 inches velocityof 100 ft/sec will succeed. A high velocity burner according to myinvention will reach a desired temperature within a shorter distancethan prior art low velocity burners of the same fuel consumption.

These good results are attibutable, it is believed, to a combination offactors. The much higher velocity (for a burner of given fuelconsumption) leads to a smaller area outlet aperture, which leads to alarger ratio of cross-section perimeter to cross-section area of thestream, which leads to a more effective pumping rate and mixing rate fora given length of the mixing zone. This can be enhanced by flatteningthe outlet area or otherwise shaping to get a large perimeter ofgas-to-air interface. As more and more cold air is drawn in, themomentum of the exhaust gases is spread over a greater air mass howeverthe velocity at the work piece remains sufficiently high to achieve goodheat transfer.

Furthermore the short time it takes for the exhaust gases from my systemto reach treating temperature means that they are not subject todetrimental buoyancy forces. Note that for slower burner outputvelocities, and slower cooling found in prior devices, the stream ofgaseous products is exposed for a quite long time to the effects of thegeneral surroundings as it moves to the work piece and it starts curvingupwards due to buoyancy forces and becomes seriously prone to beingdeflected by drafts of air, becoming uncontrollable.

I thus propose to construct a heat gun with a high velocity burner toentrain air to produce an air blast of intermediate temperature. In onepreferred embodiment the air entrainment can take place as in a free jetwith a predetermined distance between burner and work piece. Theentrainment zone preferably is within an open metal cage, one functionof which is to space the burner predictably from the work piece toassure a predetermined blast temperature delivered to the work piece andanother function is the admission of additional air along the length.The peak gas product temperature is a predictable function of spacing tothe work piece, thus an adjustable cage or other standoff or positioningmeans can be preset and precalibrated for different output temperaturerequirements.

Another preferred embodiment employs a closed mixing tube of bigger (byat least 5 time) crosssectional area than the burner outlet area. Givena sufficient length of mixing tube, generally 3 to 7 diameters, orshorter where highly dispersive burner outlets are used, thisconstruction assures complete mixing of the burner products with theentrained air resulting in high uniformity in temperature of theresulting stream. The degree of temperature attenuation, (or mixingratio) is here governed by the ratio of the mixing tube and burneroutlet cross-sectional area, and thus a desired temperature can bereproduced repeatedly by the predetermined sizing.

In preferred embodiments the outlet of the combustion chamber is shapedsuch that the outlet crosssectional area and the down streamcross-section assumes a shape with a perimeter substantially greaterthan the radius of a single circle of the same area. Such outletstypically take the shape of slits, or multiple rounds. By this means themixing length to achieve a desired temperature attenuation is reduced indirect proportion to the ambient-to-exhaust gas interface, defined bythe exposed perimeter of the stream crosssection. This behaviour can beillustrated by comparing the mixing length of two burners having thesame capacity and same exhaust gas velocity but different combustionchamber outlet configurations. An outlet that has at least 25 percentmore flow perimeter than a round outlet achieves a desired temperaturesuch as 600 F in 12 inches, contrasted with 16 inches for the circularoutlet. For success as a practical hand held burner for e.g.applications by power line repairmen, where too long a device isunwieldy, it is important that the perimeter thus be greater by 25percent than the circle of equal flow area.

Further reduction in mixing length can be achieved if the combustionchamber is shaped such that the streamlines of the exhaust gases assumea divergent pattern from the centerline, so that the perimeter of thestream cross-section downstream of the outlet is greater than at theoutlet and to separate the exhaust gas molecules from each other as muchas possible to maximize exposure to and mixing with ambient air. In thecase of multiple round outlets such a pattern can be achieved byinclining the axis of the outlet nozzles away from the center line. Inthe case of slits, such a pattern can be achieved by tapering the wallsof the combustion chamber away from the center line but maintaining aconstantly decreasing cross-sectional area of the chamber to avoiddiffusion or separation of the flow inside the chamber.

For other feature of the invention reference is made to the abstract,which is incorporated herein, and to the following description ofpreferred embodiments, the drawings and the claims.

FIG. 1 is a partially diagrammatic vertical crosssectional view of apreferred embodiment having a ducted mixing chamber and a flattened anddiverging burner outlet.

FIGS. 1a, 2, 3 and 4 are transverse cross-sections taken on lines 12, 2,3 and 4 respectively in FIG. 1.

FIG. 5 is a downward view taken on line 5 of FIG. 1.

FIG. lb is a temperature profile taken across the end of the duct.

FIG. 6 is a view similar to FIG. 1 of a second preferred embodiment;

FIG. 6a is a temperature profile taken across the end of the cage andFIGS. 7, 8 and 9 are transverse cross-sectional views taken on lines 7,8 and 9 respectively of FIG. 6.

FIG. 10 is a series of plots illustrating temperature, velocity and massflow of the embodiment of FIG. 1, with and without the duct;

FIGS. 11 and 12 are cross-sectional and end views of another preferredembodiment employing multiple burner outlet passages.

Referring to FIGS. 1-5 pressurized gas G passes through nozzle 1. Thenozzle aims into a duct 3. The nozzle-duct combination is commonly knownas a jet pump and its function is to entrain air A from openings 0around the nozzle, between struts 2, see FIG. 1a. The duct comprises afirst section of rounded form 3a, then a straight section 3b followed bydivergent section 30 and then a short length of straight section 3d. Thepump formed by rounded inlet, and subsequent straight, divergent andstraight sections provide a fuelair mixture at as high a pressure aspossible, typically 2 inches water column, up to 4 inches water column,assuming a pumping pressure of psi for gas G. Handle 4 supports the duct3 which supports all else.

The mixture is directed into the burner. The burner consists of aninternal combustion chamber 5 and the bluff body flameholder 8. Gas isburned in the combustion chamber. Flame is prevented from flashing backinto the jet pump because of the design of the flameholder. Passages,dimension e, are so small that the gas velocity therethrough is greaterthan the burning velocity so the flame simply cannot travel upstream.

At section 2 the combustion chamber is cylindrical, and then flattensout, FIG. 3. In this embodiment, with a spreading flow of the exhaustgases it is important that in the latter part of the burner, aftercombustion has occurred, the passage has equal or as shown, decreasingcross-sectional area while it fans out in one direction to increase thewetted perimeter. The flow cross-section area of FIG. 3 is larger thanthe outlet 5 FIG. 4. The gases are thus accelerated as they come out ofthe burner. The geometry is particularly important in this latter halfof the burner, to maintain velocity and avoid separation of the streamfrom the diverging walls.

In this particular burner embodiment there is first a cylindricalsection 5a less than one diameter in length from the flameholder andthen a transition section 5!) of conical form terminating less than onediameter length from the cylindrical section. From that position whichis the widest cross section of the burner, the passage Sc cross sectionarea decreases. In operation combustion initiates at the flameholder andspreads downstream.

Referring to the velocity profile FIG. 10 of air A, the air enters therounded inlet 3a at a slow velocity and speeds up to a very highvelocity inside pump 3 reaching a maximum around 8,000 fpm in section31). In the diffuser 3c it slows, the velocity energy converting tostatic pressure head. When the gas enters the burner S and is heated ittends to expand and .it increases in velocity again to a maximum in theoutlet 5 of the burner at dimension g, to around 6,000 fpm. From then onthe gas starts to entrain large quantities of treating air A and themixture slows down. At the outlet 7 the air velocity will generally begreater than feet per second, ranging from to 200 feet per second. Graphlines A represent performance of a free jet, i.e. where free flow of airoccurs into the exhaust stream at all points along jet length. Dashedlines B represent use of the closed wall tube, 7.

In effect the hot gases from the burner 5 drive a second jet pump, topump, mix and heat ambient air A The duct 7 of this second jet pump inFIG. 1 has a cross section area which as is shown in FIG. 4 issubstantially larger than the outlet area 5 of the burner, with an orderof magnitude from 5 to 50. The air inlet 70 to duct 7 is ofcorresponding size, due to its flared form, positioned by struts 6concentrically about the burner 5. The velocity of the hot gasesentrains cold air, and this stream mixes in the duct, the reason for theduct being to equalize the velocities and the temperature of themixture. If the duct were cut too short a hot core and a cold outsidewould be found. Complete mixing occurs so that after a length of morethan about 3 diameters up to 7 depending upon design, equal temperatureand equal velocity come out. The amount of air entrained is governedprimarily by the area ratio of duct 7 to the burner outlet area.

In a typical construction in accordance with the embodiment of FIG. 1the dimensions may be selected as follows:

a 0.0187 in. 11 0.250 in.

b 0.235 in. 1 L500 in.

c 0.575 in. 1 2.800 in.

d 0.670 in. 1 L000 in.

e 0.134111. 1, 1.500 in.

f 0.890 in. 1 4.000 in.

g 0.340 in.

I: 1.300 in. Angle at 7 1' 1.250 in. 8 20 j 3.500 in.

With this particular construction with propane introduced at 22% psig,at a fuel rate of 0.0116 lb/min, corresponding to 13,500 BTU/hour, thehot air gun will deliver 28.6 cfm of air at 1,000 F, velocity 1,200 fpm.

In'accordance with the embodiment of FIG. 6 the mixing process can beobtained without the closed-wall duct in what is called a free jet inwhich air can enter the mixing stream at any point downstream. FIG. 6has the same jet pump in common as FIG. 1. It shows different burnergeometry 9. The burner in FIG. 6 has three outlet slits 9 arranged inclover-leaf formation rather than one slit, with transition fromcylindrical to that form, compare FIGS. 7, 8 and 9. Again the first halfof the combustion chamber shape or cross sectional area is not criticalbut the latter half has ever decreasing cross-sectional areas. On thebasis of wanting at least 75 feet per second outlet gas velocity, thecrosssections become calculable after the heating capacity isestablished. A 15,000 BTU/hour burner would typically have a 3/ 10square inch of outlet area, FIG. 7, arrived at taking into considerationthe maximum velocity of the generator and the amount of combustiongases.

The mixing of FIG. 6 is very length-dependent. The further downstreamfrom the burner the more air is drawn in, the lower the temperature hasdropped, see FIG. 10. This temperature attenuation curve is verypredictable for each size of outlet and velocity through it. Due to thefact that there is an ever decreasing temperature, one can select thetemperature wanted. To assure constant spacing, a device such as shownin FIG. 6 is employed where cage 11 serves to position the burnerrelative to the workpiece andstill admit air for mixing.This cage can beadjusted. It consists of a strut 12 that mounts in a support post 13. Itis fixed with a screw 14. This strut has indentations shown so it can becalibrated for various temperatures. This whole structure holds the cagein the calibrated position relative to the burner.

In the embodiment of FIG. 6 one needs to move the cage back and forth toestablish the heated air temperature at the end of the cage. It sets amaximum temperature deliverable to the workpiece and by moving away onecan set lower temperatures. Typically, to achieve the same lowtemperature with same burner design, the length of cage 11 beyond theburner will be less than the length of tube 7, and hence may be moreconvenient for certain applications. Where the uniformity of thetemperature of all air emitted from the outlet is important, one maychoose however the embodiment of FIG. I over that of FIG. 6, compareFIGS. 1b and 6a and 10. Another advantage of FIG. 1 is that it iswindproof in high cross winds, useful for instance in airports,railroads and power and telephone line repair.

Referring to FIGS. 10 and 11 another embodiment employs chamber outletopenings like a shower head, with axes of openings divergent from oneanother to provide a downwardly expanding stream.

Numerous other embodiments will be recognized to be within the scope ofthe invention.

I claim:

1. A hand held gun for providing a flow of heated air in the 250 F toL000 F range against a work object relying upon fuel alone withoutassistance of blowers or compressors, said gun comprising thecombination of a gaseous fuel jet adapted for connection to aconventional fuel gas source such as propane having a stoichiometricburning temperature above 3,000 F, a jet pump activated by said gas jetand having an opening for drawing atmospheric air for combustion into asubatmospheric pressure region produced by said jet, said 5 jet pumpconstructed to impart velocity to said combustion air by mixing, anenlarged pressure recovery passage into which the mixture of gaseousfuel and combustion air proceeds, said recovery passage constructed toconvert velocity head of said gases to a pressure head exceedingatmospheric pressure, an internal combustion chamber, said chamberhaving an entry into which said pressure recovery passage discharges, aflame holding means at said entry and an outlet discharging combustiongases, the effective wetted perimeter of the flow cross-section of saidoutlet being at least 25 percent greater in length than the perimeter ofa single circle of identical cross-sectional area, providing an extendedinterface between combustion gases discharging from said outlet andatmospheric air, and a temperature-limiting structure extendingdownstream of said outlet, said structure preventing direct access ofthe work object to said outlet and to the combustion gases emittingtherefrom and defining an ambient air propelling and mixing zone throughwhich said combustion gases flow preceding said work object, saidtemperature-limiting structure being open to admit atmospheric airfreely, progressively to the stream of combustion gases as they proceedthrough said structure, the respective parts of said gun constructed toburn said fuel in substantially stoichiometric conditions and dischargesaid combustion gases through said outlet at a temperature exceeding3,000 F and a velocity in excess of 4,000 feet per minute, said gun,through the cooperation of said extended combustion gas-air interface,effectively producing a flow comprised in major part of ambient airpropelled and heated by said combustion gases, the effective length ofsaid temperature-limiting structure downstream of said outletdetermining the temperature of the resultant flow at the output end.

2. The gun of claim I having a multiplicity of elongated outlet apertureportions for combustion gases, and aperture portions arranged todischarge in the same general direction into different portions of saidmixing and propelling zone.

3. The gun of claim 1 wherein said temperature limiting structurecomprises a shield member extending about said outlet in a spacedrelation defining an axial inlet for atmospheric air adjacent saidoutlet and having wall portions extending downstream providing amultiplicity of lateral atmospheric air openings along the length ofsaid temperature-limiting structure through which air may freely entersaid shield, there to be propelled and heated by said combustion gases.

4. The gun of claim 1 including means fixing the downstream end of saidtemperature-limiting device at a predetermined position from said outletthereby for a given fuel flow, enabling the temperature at saiddownstream end to be set at a predetermined level.

5. A hand held gun for providing a flow of heated air in the 250 F to1,000 F range against a work object relying upon fuel alone withoutassistance of blowers or compressors, said gun comprising thecombination of a gaseous fuel jet adapted for connection to aconventional fuel gas source such as propane having a stoichiometricburning temperature above 3.000" F, a jet pump activated by said gas jetand having an opening for drawing atmospheric air for combustion into asubatmospheric pressure region produced by said jet, said jet pumpconstructed to impart velocity to said combustion air by mixing, anenlarged pressure recovery passage into which the mixture of gaseousfuel and combustion air proceeds, said recovery passage constructed toconvert velocity head of said gases to a pressure head exceedingatmospheric pressure, an internal combustion chamber, said chamberhaving an entry into which said pressure recovery passage discharges, aflame holding means at said entry and an outlet discharging combustiongases, the effective wetted perimeter of the flow cross-section of saidoutlet being at least 25 percent greater in length than the perimeter ofa single circle of identical cross-sectional area, providing an extendedinterface between combustion gases discharging from said outlet andatmospheric air, and a temperature-limiting structure extendingdownstream of said outlet, said structure preventing direct access ofthe work object to said outlet and to the combustion gases emittingtherefrom and defining an ambient air propelling and mixing zone throughwhich said combustion gases flow preceding said work object, saidtemperature -limiting structure comprising an elongated tube extendingabout said outlet in a spaced relation defining an axial inlet forinduced flow of atmospheric air adjacent said outlet, said tube having aclosed wall along its length and the flow cross-section and capacity ofsaid air inlet and said tube being more than five times greater than theflow cross-section and flow capacity of said burner, the respectiveparts of said gun constructed to burn said fuel in substantiallystoichiometric conditions and discharge said combustion gases throughsaid outlet at a temperature exceeding 3,000 F and a velocity in excessof 4,000 feet per minute, said gun through the cooperation of saidextended combustion gas-air interface effectively producing a flowcomprised in major part of ambient air propelled and heated by saidcombustion gases, the effective flow capacity of said air inletdetermining the temperature of the resultant flow at the output end.

6. The head gun of claim 1 wherein said burner has a constant ordecreasing flow cross-section area leading to said outlet.

7. The head gun of claim 1 wherein said outlet comprises an elongatedoutlet aperture, said burner having walls diverging in the direction ofelongation of said aperture.

UNl'lED STATES PATENT ommee CERTlFlQATE QQRREQTEQN Patent No. 73,779,694 Dated December 18, 1973 Inventor(s) Dimiter S. Zaqoroff It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

In the abs-tract, first page, second colummline 3,

delete the word e Column 1 line 45 10w should be lower- Column l, line.62, "attibutable" should be --at'tributable--;

Column 3, line 19, "feature should be -features";

Column 6, line 43, and should be said Column 8, line l7 "head" shoulelbe heat;

Column 8, line 20, "head" should be "hea't Signed and eeeled ehie letday of Gmzober 1974.

(SEAL) Attest':

MCCOY Ma GIBSON JR, Ce MARSlLkLL DANN Attesting Gfflcer Conmxlselener ofPatents FORM PO-lOSO (IO-69) UsCQMM-Dc 603754959 1: us. GOVERNMENTPRINTING OFFICE 1959 o-ass-au.

1. A hand held gun for providing a flow of heated air in the 250* F to1,000* F range against a work object relying upon fuel alone withoutassistance of blowers or compressors, said gun comprising thecombination of a gaseous fuel jet adapted for connection to aconventional fuel gas source such as propane having a stoichiometricburning temperature above 3,000* F, a jet pump activated by said gas jetand having an opening for drawing atmospheric air for combustion into asubatmospheric pressure region produced by said jet, said jet pumpconstructed to impart velocity to said combustion air by mixing, anenlarged pressure recovery passage into which the mixture of gaseousfuel and combustion air proceeds, said recovery passage constructed toconvert velocity head of said gases to a pressure head exceedingatmospheric pressure, an internal combustion chamber, said chamberhaving an entry into which said pressure recovery passage discharges, aflame holding means at said entry and an outlet discharging combustiongases, the effective wetted perimeter of the flow cross-section of saidoutlet being at least 25 percent greater in length than the perimeter ofa single circle of identical cross-sectional area, providing an extendedinterface between combustion gases discharging from said outlet andatmospheric air, and a temperature-limiting structure extendingdownstream of said outlet, said structure preventing direct access ofthe work object to said outlet and to the combustion gases emittingtherefrom and defining an ambient air propelling and mixing zone throughwhich said combustion gases flow preceding said work object, saidtemperature-limiting structure being open to admit atmospheric airfreely, progressively to the stream of combustion gases as they proceedthrough said structure, the respective parts of said gun constructed toburn said fuel in substantially stoichiometric conditions and dischargesaid combustion gases through said outlet at a temperature exceeding3,000* F and a velocity in excess of 4,000 feet per minute, said gun,through the cooperation of said extended combustion gas-air interface,effectively producing a flow comprised in major part of ambient airpropelled and heated by said combustion gases, the effective length ofsaid temperature-limiting structure downstream of said outletdetermining the temperature of the resultant flow at the output end. 2.The gun of claim 1 having a multiplicity of elongated outlet apertureportions for combustion gases, and aperture portions arranged todischarge in the same general direction into different portions of saidmixing and propelling zone.
 3. The gun of claim 1 wherein saidtemperature limiting structure comprises a shield member extending aboutsaid outlet in a spaced relation defining an axial inlet for atmosphericair adjacent said outlet and having wall portions extending downstreamproviding a multiplicity of lateral atmospheric air openings along thelength of said temperature-limiting structure through which air mayfreely enter said shield, there to be propelled and heated by saidcombustion gases.
 4. The gun of claim 1 including means fixing thedownstream end of said temperature-limiting device at a predeterminedposition from said outlet thereby for a given fuel flow, enabling thetemperature at said downstream end to be set at a predetermined level.5. A hand held gun for providing a flow of heated air in the 250* F to1,000* F range against a work object relying upon fuel alone withoutassistance of blowers or compressors, said gun comprising thecombination of a gaseous fuel jet adapted for connection to aconventional fuel gas source such as propane having a stoichiometricburning temperature above 3,000* F, a jet pump activated by said gas jetand having an opening for drawing atmospheric air for combustion into asubatMospheric pressure region produced by said jet, said jet pumpconstructed to impart velocity to said combustion air by mixing, anenlarged pressure recovery passage into which the mixture of gaseousfuel and combustion air proceeds, said recovery passage constructed toconvert velocity head of said gases to a pressure head exceedingatmospheric pressure, an internal combustion chamber, said chamberhaving an entry into which said pressure recovery passage discharges, aflame holding means at said entry and an outlet discharging combustiongases, the effective wetted perimeter of the flow cross-section of saidoutlet being at least 25 percent greater in length than the perimeter ofa single circle of identical cross-sectional area, providing an extendedinterface between combustion gases discharging from said outlet andatmospheric air, and a temperature-limiting structure extendingdownstream of said outlet, said structure preventing direct access ofthe work object to said outlet and to the combustion gases emittingtherefrom and defining an ambient air propelling and mixing zone throughwhich said combustion gases flow preceding said work object, saidtemperature -limiting structure comprising an elongated tube extendingabout said outlet in a spaced relation defining an axial inlet forinduced flow of atmospheric air adjacent said outlet, said tube having aclosed wall along its length and the flow cross-section and capacity ofsaid air inlet and said tube being more than five times greater than theflow cross-section and flow capacity of said burner, the respectiveparts of said gun constructed to burn said fuel in substantiallystoichiometric conditions and discharge said combustion gases throughsaid outlet at a temperature exceeding 3,000* F and a velocity in excessof 4,000 feet per minute, said gun through the cooperation of saidextended combustion gas-air interface effectively producing a flowcomprised in major part of ambient air propelled and heated by saidcombustion gases, the effective flow capacity of said air inletdetermining the temperature of the resultant flow at the output end. 6.The head gun of claim 1 wherein said burner has a constant or decreasingflow cross-section area leading to said outlet.
 7. The head gun of claim1 wherein said outlet comprises an elongated outlet aperture, saidburner having walls diverging in the direction of elongation of saidaperture.